I guess I would have to vote (no pun intended) for the American system. It has worked reasonably well for over 200 years (although the Civil War brought to a head some major flaws in the original design). Germany's democratic government, on the other hand, is not nearly as old or as tested. The United Kingdom is a nation I truly venerate for all kinds of reasons, not the least of which is that the British Parliament is "the mother of parliaments." Still, I like the fact that I live in a country that does indeed have a written constitution, as Britain does not (or at least not in the sense that ours is written). I admire all three of the countries you have mentioned, and I can see much to admire about the political systems of all three.
I will have to go with the US. I have lived in the UK and have a lot of friends who immigrated from Germany to the US. I enjoyed thoroughly my life in the UK and my German friends always have wonderful things to say about their motherland. Yet, there is one thing we all have in common: We all ended up here in the US.
I feel it is not necessarily because of the government, as I personally did not vote for our current President NOR agree with PLENTY of his policies and ideas. It is more because I feel that we have more variety in the US as far as regions where we could live, and we can be more financially free to move around here. This is funny because in the UK I would go from Oxfordshire to Yorkshire, then from Cornwall to Berwick UT and, yes, it is as varied as going from Maine to Texas, or Hawaii to Minnesota. However, here is way less expensive than in the UK or Germany. Also, I personally feel more connected with the cultural varieties that I see in the US than the other cultures I befriended while I lived overseas.
I think you are going to find it very hard to receive an unbiased response here! For me, as a Brit, there are lots of things I like about the British system of government. You might want to think about the difficulties that the US have because they have a constitution, especially concerning an issue such as the right to own firearms. Having constitutionally enshrined rights means that it is very difficult to take those rights away. For me, Britain's system of law allows it to be more flexible in this instance.
This will be completely subjective and opinion based. I am biased enough to say that I prefer the United States system of government for a couple of reasons. The first would be that the Constitution that governs the United States political sphere is probably the strongest document that speaks to a democracy. Germany and England have their constitutions, but I don't think each nation's document is as expansive and as pervasive as our Constitution in terms of speaking to as many different parts of government as possible. The amendments are protections for individuals from government and the principles and goals of the Constitution have been in place for the last two centuries with relative calm. While on its face value there is some fundamental tension between the economic system of capitalism and the democratic system of government, for the most part, America has made it work, showing that capitalism can be malleable and can be modified to the needs of its citizens. Finally, I think that the United States government has done a decent job of consistently speaking out for the rights of those who are not represented. There have been problems, but not as many as one could have seen for the last two centuries. The Constitution's fundamental goal of "forming a more perfect union" has spoken to the idea of American democracy being an unfinished experiment. To this end, I would say that the United States government has proven to have the best system of government. Yet, in all honesty, this is going to be a debate that will expand from even the choices mentioned.